In various applications, it is necessary to remove accumulations of materials that impair the proper functioning of the installation.
Such is the case in cement works or bulk materials silos that are emptied by gravity.
To do this, instead of using purely mechanical means like a metal bar manipulated by an individual, it is known to use air blast devices.
The principle of these devices consists of filling a reservoir or a container with air at a given pressure and to allow the air to escape suddenly so as to produce a blast.
Under the effect of the blast, the accumulations of materials are broken up.
The advantage of these devices consists in the fact that they can function automatically and be disposed in places that are not readily accessible.
The known devices comprise a device for controlling the flow of a gaseous fluid established on a course linking the gaseous fluid accumulator and possibly an ejection nozzle.
Thus, conventionally, the blast device comprises a body housing a piston whose front face closes a so-called outlet port that opens into an outlet conduit, this body having an inlet conduit that connects it to the reservoir or container.
Throughout the filling period of the container, the rear face of this piston is subjected to a pressure that holds the piston over the outlet.
When the pressure maintaining the pressure on the piston is released, the latter suddenly moves back and allows the fluid to pass from the container to the outlet conduit, which may or may not incorporate a nozzle.
In the devices known to date, the outlet conduit has a constant cross-section along its longitudinal axis identical to that of the port between the container and the body of the device.
However, quite often, the geometry of the outlet conduit is different along the axis, i.e., at the level of the area to which the piston is applied, the cross-section of the outlet conduit is circular, sometimes evolving into an ellipse.
The efficiency of these devices is directly linked to the discharge speed of the air contained in the reservoir.
This ejection speed specifically depends on the opening speed of the piston.
Thus, we conceived of making the piston lighter.
To do this, the latter is given the shape of a cup, particularly a truncated one, whose convex front face closes the outlet port and whose concave rear face supports a means for guiding it with the body of the device.
This guiding means is preferably reduced to a slider that moves inside a guide attached to rear of the body.
This makes it possible both to make the piston lighter and to reduce friction by reducing the guide surfaces.
Particularly for purposes of installing these blast devices, it is convenient for the inlet conduit that connects the fluid flow control device to the container to be approximately perpendicular to the outlet conduit.
This also makes it possible to limit the distance between the inlet and the outlet.
Along its course, there is necessarily a loss of pressure that reduces the efficiency of the device.
It is of course possible to increase the efficiency of these devices by increasing the volumetric capacity of the reservoir or by increasing the pressure of the air contained in the reservoir, but this is not always possible, either for economic reasons or because of the installation of the blast devices.
Another problem results from the noise produced by the evacuation of the air contained in the rear chamber.
In order to reduce the noise during the emptying of the rear chamber, it is thus known to evacuate the air contained in this chamber, either directly or indirectly, into the outlet conduit.
To do this, it is known from U.S. Pat. No. 4,201,362 to equip the front face of the piston with a series of valve disposes one behind the other.
Such an assembly results in an increase in the weight of the main valve, and hence a loss of efficiency in the device.